> Inventory:
> 1 Nanoxite Stone
> Passcard (CQ60537)
> Light Tube
> ISO Officer's Cap
> Small Mirror
> Leather Briefcase
> Supply Room Card
> Radio Receiver
> Pet Snacks
> Ruby
> Employment Papers
> Mine Access Card
> Fire Extinguisher
> Matches
> I.D. Card
> OSA Card
> Crank
> Washcloth
> Passcard (CQ30021)
> Business Card
> Pickaxe
> Plastic Cup
> Passcard (CQ40844)
> Crowbar
> Noise Filter Headset
> Round Pot
>
> I started doing the OSA tubes at Kepler's office, as advised, and then once I got the password entered there, I went and searched out the other two accesses, fended off some beetles, and entered in the passwords, but now I'm (according to Johnson) supposed to go back to M-30 and press the reset switch. But I can't seem to get back into the 30 section of the OSA, just the 20 and 10. How am I supposed to get back into the 30 section?

At any point (with an exception I'll get to), you can leave the section you're in and enter just one of the other two.

The pattern is that if you want to go from 30 -> 10, or 10 -> 20, or 20 -> 30, you need to start in the control room corresponding to the first number, then if necessary use the emergency switch so that you can exit through the north end of the tube (into one of the docking bays).

If the switch is set oppositely, you instead exit through one of the doors along the way. This is how you'd need it to be if you wanted to go 10 -> 30, or 20 -> 10, or 30 -> 20 (I just reversed the directions from the last paragraph).

So for now it looks like you have 10 -> 20 and 20 -> 10 available. The game solution involves going 30 -> 10 -> 20 (in this order) to reach all the control rooms. So, in your case, one way you could get back would be to reverse the steps you used to get to the last section. You'd go to control room M-10, then use the emergency switch once, changing the access from 10 -> 20 to 10 -> 30. Then exit the 10 section and find a working door that lets you enter the 30 section. The end result is that you take the path 30 -> 10 -> 20 -> 10 -> 30 to solve the puzzle.

The other way is to find a way to go directly between 20 and 30. In other words, the quickest solution is to visit 30 -> 10 -> 20 -> 30, with the first three numbers being places you put in the security code. But there's a problem with one of the doors in the 20 section. If you can figure out a workaround, you should be able to see how to go between 20 and 30 in one leg.